Alcohol abuse is a national problem that extends into virtually all aspects of society. Current practice for alcohol measurements to detect alcohol abuse is based upon either blood measurements or breath testing. Blood measurements define the gold standard for determining alcohol intoxication levels. However, blood measurements require either a venous or capillary sample and involve significant handling precautions in order to minimize health risks. Once extracted, the blood sample must be properly labeled and transported to a clinical laboratory or other suitable location where a clinical gas chromatograph is typically used to measure the blood alcohol level. Due to the invasiveness of the procedure and the amount of sample handling involved, blood alcohol measurements are usually limited to critical situations such as for traffic accidents, violations where the suspect requests this type of test, and accidents where injuries are involved.
Because it is less invasive, breath testing is more commonly encountered in the field. In breath testing, the subject must expire air into the instrument for a sufficient time and volume to achieve a stable breath flow that originates from the alveoli deep within the lungs. The device then measures the alcohol content in the air, which is related to blood alcohol through a breath-blood partition coefficient. The blood-breath partition coefficient used in the United States is 2100 (implied units of mg EtOH/dL blood per mg EtOH/dL air) and varies between 1900 and 2400 in other nations. The variability in the partition coefficient is due to the fact that it is highly subject dependent. In other words, each subject will have a partition coefficient in the 1900 to 2400 range that depends on his or her physiology. Since knowledge of each subject's partition coefficient is unavailable in field applications, each nation assumes a single partition coefficient value that is globally applied to all measurements. In the U.S., defendants in DUI cases often use the globally applied partition coefficient as an argument to impede prosecution.
Breath measurements have additional limitations. First, the presence of “mouth alcohol” can falsely elevate the breath alcohol measurement. This necessitates a 15-minute waiting period prior to making a measurement in order to ensure that no mouth alcohol is present. For a similar reason, a 15 minute delay is required for individuals who are observed to burp or vomit. A delay of 10 minutes or more is often required between breath measurements to allow the instrument to return to equilibrium with the ambient air and zero alcohol levels. In addition, the accuracy of breath alcohol measurements is sensitive to numerous physiological and environmental factors.
Multiple government agencies, and society in general, seek noninvasive alternatives to blood and breath alcohol measurements. Quantitative spectroscopy offers the potential for a completely noninvasive alcohol measurement that is not sensitive to the limitations of the current measurement methodologies. While noninvasive determination of biological attributes by quantitative spectroscopy has been found to be highly desirable, it has been very difficult to accomplish. Attribute properties of interest include, as examples, analyte presence, analyte concentration (e.g., alcohol concentration), direction of change of an analyte concentration, rate of change of an analyte concentration, disease presence (e.g., alcoholism), disease state, and combinations and subsets thereof. Noninvasive measurements via quantitative spectroscopy are desirable because they are painless, do not require a fluid draw from the body, carry little risk of contamination or infection, do not generate any hazardous waste, and can have short measurement times.
Several systems have been proposed for the noninvasive determination of attribute properties of biological tissue. These systems have included technologies incorporating polarimetry, mid-infrared spectroscopy, Raman spectroscopy, Kromoscopy, fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, radio-frequency spectroscopy, ultrasound, transdermal measurements, photo-acoustic spectroscopy, and near-infrared spectroscopy. However, these systems have not replaced direct and invasive measurements.
As an example, Robinson et al. in U.S. Pat. No. 4,975,581 disclose a method and apparatus for measuring a characteristic of unknown value in a biological sample using infrared spectroscopy in conjunction with a multivariate model that is empirically derived from a set of spectra of biological samples of known characteristic values. The above-mentioned characteristic is generally the concentration of an analyte, such as alcohol, but also can be any chemical or physical property of the sample. The method of Robinson et al. involves a two-step process that includes both calibration and prediction steps.
In the calibration step, the infrared light is coupled to calibration samples of known characteristic values so that there is attenuation of at least several wavelengths of the infrared radiation as a function of the various components and analytes comprising the sample with known characteristic value. The infrared light is coupled to the sample by passing the light through the sample or by reflecting the light off the sample. Absorption of the infrared light by the sample causes intensity variations of the light that are a function of the wavelength of the light. The resulting intensity variations at a minimum of several wavelengths are measured for the set of calibration samples of known characteristic values. Original or transformed intensity variations are then empirically related to the known characteristics of the calibration samples using multivariate algorithms to obtain a multivariate calibration model. The model preferably accounts for subject variability, instrument variability, and environment variability.
In the prediction step, the infrared light is coupled to a sample of unknown characteristic value, and a multivariate calibration model is applied to the original or transformed intensity variations of the appropriate wavelengths of light measured from this unknown sample. The result of the prediction step is the estimated value of the characteristic of the unknown sample. The disclosure of Robinson et al. is incorporated herein by reference.
A further method of building a calibration model and using such model for prediction of analytes and/or attributes of tissue is disclosed in commonly assigned U.S. Pat. No. 6,157,041 to Thomas et al., titled “Method and Apparatus for Tailoring Spectrographic Calibration Models,” the disclosure of which is incorporated herein by reference. In U.S. Pat. No. 5,830,112, Robinson describes a general method of robust sampling of tissue for noninvasive analyte measurement. The sampling method utilizes a tissue-sampling accessory that is pathlength optimized by spectral region for measuring an analyte, such as alcohol. The patent discloses several types of spectrometers for measuring the spectrum of the tissue from 400 to 2500 nm, including acousto-optical tunable filters, discrete wavelength spectrometers, filters, grating spectrometers and FTIR spectrometers. The disclosure of Robinson is incorporated hereby reference.
Spectroscopic measurements offer promise for completely noninvasive alcohol measurements in people in controlled environments. In U.S. Pat. No. 5,743,349, titled “Non-invasive optical blood alcohol concentration reader and vehicle ignition interlock system and method”, Steinberg discloses a vehicle ignition interlock that incorporates a spectroscopic means for noninvasively measuring blood alcohol concentration. Steinberg does not disclose any means for verifying the identity of the tested individual. Furthermore, Steinberg discloses the measurement of electromagnetic radiation in the 250 to 3000 nm wavelength range by introducing radiation to a finger and measuring the light exiting the opposite side of the finger. Such transmission approaches, while potentially feasible in the visible region (400 to 800 nm), are limited by the strong absorption of water (water is a major component of the tissue) in the near- and mid-infrared regions (>800 nm). For tissue samples greater than a few millimeters in thickness, the absorption of water results in virtually no measurable radiation exiting the opposite side of the sample. Consequently, little if any radiation remains for subsequent measurement of alcohol concentration.
In U.S. Pat. No. 6,229,908, titled “Driver Alcohol Ignition Interlock”, Edmonds and Hopta disclose an ignition interlock incorporating a spectroscopic alcohol measurement of the finger combined with a means for generating a finger print image. The finger print image is intended to identify the operator in order to ensure that the alcohol measurement was acquired from the prospective driver and not a passenger. Similar to existing breath-based interlocks, the finger print image is obtained from a measurement that is distinct from the spectroscopic measurement, thereby yielding potential for circumventing the interlock. Further, the method disclosed by Edmonds and Hopta relies on automated fingerprint reading, a technology which has demonstrated performance shortcomings.
Therefore, although there has been substantial work conducted in attempting to produce commercially viable noninvasive near-infrared spectroscopy-based systems for determination of biological attribute properties, including the presence of alcohol, or substances of abuse, no such device is presently available. It is believed that prior art systems discussed above have failed for one or more reasons to fully meet the challenges imposed by the spectral characteristics of tissue which make the design of a noninvasive measurement system a formidable task. Thus, there is a substantial need for a commercially viable device which incorporates subsystems and methods with sufficient accuracy and precision to make clinically relevant determinations of biological attribute properties in human tissue.